It's probably not that something in particular would've caused loss of interest, but that the hype of it being a new thing was what got so many players into it at first. Things like that aren't very likely to maintain or get even more people as they did in the hype rush of the first few days/weeks.
Interest for stuff like that also seems to come and go in waves. If a group of people goes back for a while they'll probably pull some more people with them. Sometimes when you log in you might actually see half the players over at skyblock because someone decided to take a break from ordinary survival.